This invention relates to an aqueous composition suitable for use when dry as an improved coating, a method for preparing an aqueous emulsion polymer suitable for use in an improved coating and methods for providing a coated substrate and for improving the scrub resistance of a coating. More particularly this invention relates to an aqueous composition including a aqueous emulsion polymer, the polymer having a glass transition temperature (Tg) from greater than 20xc2x0 C. to 80xc2x0 C., the polymer formed by the free radical polymerization of at least one ethylenically unsaturated nonionic acrylic monomer and 0-7.5%, by weight based on the total weight of the polymer, ethylenically unsaturated acid monomer, in the presence of 0.01-1.0%, by weight based on the total weight of the polymer, t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms and, optionally, at least one other oxidant, a method of preparing the emulsion polymer, a method for providing a coated substrate wherein the coating contains the emulsion polymer having a Tg from xe2x88x9220xc2x0 C. to 80xc2x0 C., and a method for improving the scrub resistance of a coating by including the emulsion polymer having a Tg from xe2x88x9220xc2x0 C. to 80xc2x0 C.
The present invention serves to provide an aqueous composition suitable for use when dry as an improved coating, by which is meant that the coating, xe2x80x9ccoatingxe2x80x9d herein including, for example, paint, clearcoat, topcoat, primer, paper coating, and leather coating, expressly excluding elastomeric coating, caulk, sealant, and pressure sensitive adhesive, exhibits improvement in at least one of scrub resistance, marker stain blocking, corrosion resistance over metal, flash rust resistance over metal, gloss(higher), exterior durability as indicated, for example, by gloss retention or cracking resistance, adhesion to substrates, water vapor permeability, and water swelling, relative to a coating in which an emulsion polymer of the same Tg not so formed is employed.
U.S. Pat. No. 5,540,987 discloses emulsion polymers including at least 50% vinyl acetate having low residual formaldehyde and providing saturated cellulosic webs having improved tensile strength. The polymers are formed by the use of an hydrophobic hydroperoxide and ascorbic acid initiator throughout the course of the reaction.
The problem faced by the inventors is the provision of an aqueous composition suitable for use when dry as an improved coating. Unexpectedly, the inventors found that the use of certain levels of t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms during the polymerization or even only in the last stages of the polymerization was sufficient to provide emulsion polymers which led to improved coatings properties.
In a first aspect of the present invention there is provided an aqueous coating composition comprising an aqueous emulsion polymer, the polymer having a glass transition temperature (Tg) from greater than 20xc2x0 C. to 80xc2x0 C., formed by the free radical polymerization of at least one ethylenically unsaturated nonionic acrylic monomer and 0-7.5%, by weight based on the total weight of the polymer, ethylenically unsaturated acid monomer in the presence of 0.01-1.0%, by weight based on the total weight of the polymer, t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms.
In a second aspect of the present invention there is provided a method for preparing an aqueous emulsion polymer suitable for use in a coating composition comprising forming an aqueous emulsion polymer, the polymer having a glass transition temperature (Tg) from greater than 20xc2x0 C. to 80xc2x0 C., formed by the free radical polymerization of at least one ethylenically unsaturated nonionic acrylic monomer and 0-7.5%, by weight based on the total weight of the polymer, ethylenically unsaturated acid monomer in the presence of 0.01-1.0%, by weight based on the total weight of the polymer, t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms.
In a third aspect of the present invention there is provided a method for providing a coated substrate comprising forming an aqueous coating composition comprising an aqueous emulsion polymer, the polymer having a glass transition temperature (Tg) from xe2x88x9220xc2x0 C. to 80xc2x0 C., formed by the free radical polymerization of at least one ethylenically unsaturated nonionic acrylic monomer and 0-7.5%, by weight based on the total weight of the polymer, ethylenically unsaturated acid monomer in the presence of 0.01-1.0%, by weight based on the total weight of the polymer, t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms; applying the aqueous coating composition to the substrate; and drying, or allowing to dry, the aqueous composition.
In other aspects of the present invention there are provided a method for improving the scrub resistance of a coating, a method for improving the outdoor durability of a coating, and a method for improving the pick strength of a paper or paperboard coating.
This invention relates to an aqueous composition suitable for use when dry as a coating including an aqueous emulsion polymer, the polymer having a glass transition temperature (Tg) from greater than 20xc2x0 C. to 80xc2x0 C., formed by the free radical polymerization of at least one ethylenically unsaturated nonionic acrylic monomer and 0-7.5%, by weight based on the total weight of the polymer, ethylenically unsaturated acid monomer in the presence of 0.01-1.0%, by weight based on the total weight of the polymer, t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms and, optionally, at least one other oxidant.
The aqueous emulsion polymer contains at least one copolymerized ethylenically unsaturated nonionic acrylic monomer. By xe2x80x9cnonionic monomerxe2x80x9d herein is meant that the copolymerized monomer residue does not bear an ionic charge between pH=1-14.
The ethylenically unsaturated nonionic acrylic monomers include, for example, (meth)acrylic ester monomers including methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, lauryl acrylate, methyl methacrylate, butyl methacrylate, isodecyl methacrylate, lauryl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate. Other ethylenically unsaturated nonionic monomers which may be incorporated into the polymer include, for example, styrene and substituted styrenes; butadiene; vinyl acetate, vinyl butyrate and other vinyl esters; vinyl monomers such as vinyl chloride, vinyl toluene, and vinyl benzophenone; and vinylidene chloride. Preferred are all-acrylic, styrene/acrylic, and vinyl acetate/acrylic polymers. Preferred is a predominantly acrylic aqueous emulsion polymer. By xe2x80x9cpredominantly acrylicxe2x80x9d herein is meant that the polymer contains greater than 50%, by weight, copolymerized units deriving from (meth)acrylic monomers such as, for example, (meth)acrylate esters, (meth)acrylamides, (meth)acrylonitrile, and (meth)acrylic acid. The use of the term xe2x80x9c(meth)xe2x80x9d followed by another term such as acrylate or acrylamide, as used throughout the disclosure, refers to both acrylates or acrylamides and methacrylates and methacrylamides, respectively.
The emulsion polymer contains from 0% to 7.5%, by weight based on total monomer weight, of a copolymerized monoethylenically-unsaturated acid monomer, based on the weight of the polymer, such as, for example, acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, monomethyl itaconate, monomethyl fumarate, monobutyl fumarate, maleic anhydride, 2-acrylamido-2-methylpropane sulfonic acid, vinyl sulfonic acid, styrene sulfonic acid, 1-allyloxy-2-hydroxypropane sulfonic acid, alkyl allyl sulfosuccinic acid, sulfoethyl (meth)acrylate, phosphoalkyl (meth)acrylates such as phosphoethyl (meth)acrylate, phosphopropyl (meth)acrylate, and phosphobutyl (meth)acrylate, phosphoalkyl crotonates, phosphoalkyl maleates, phosphoalkyl fumarates, phosphodialkyl (meth)acrylates, phosphodialkyl crotonates, and allyl phosphate.
The emulsion polymer used in this invention may contain from 0% to 1%, by weight based on monomer weight, copolymerized multi-ethylenically unsaturated monomers such as, for example, allyl methacrylate, diallyl phthalate, 1,4-butylene glycol dimethacrylate, 1,2-ethylene glycol dimethacrylate, 1,6-hexanediol diacrylate, and divinyl benzene.
The glass transition temperature (xe2x80x9cTgxe2x80x9d) of the emulsion polymer is from is measured by differential scanning calorimetry (DSC) taking the mid-point in the heat flow versus temperature transition as the Tg value, the monomers and amounts of the monomers being selected to achieve the desired polymer Tg range as is well known in the art. The preferred Tg of the emulsion polymer described hereinabove for use in coatings is from xe2x88x9220xc2x0 C. to 80xc2x0 C., more preferably from 0xc2x0 C. to 40xc2x0 C.
The polymerization techniques used to prepare aqueous emulsion-polymers are well known in the art. In the emulsion polymerization process conventional surfactants may be used such as, for example, anionic and/or nonionic emulsifiers such as, for example, alkali metal or ammonium salts of alkyl, aryl, or alkylaryl sulfates, sulfonates or phosphates; alkyl sulfonic acids; sulfosuccinate salts; fatty acids; ethylenically unsaturated surfactant monomers; and ethoxylated alcohols or phenols. The amount of surfactant used is usually 0.1% to 6% by weight, based on the weight of monomer. Either thermal or redox initiation processes may be used. The reaction temperature is maintained at a temperature lower than 120xc2x0 C. throughout the course of the reaction. Preferred is a reaction temperature between 30xc2x0 C. and 95xc2x0 C., more preferably between 50xc2x0 C. and 90xc2x0 C. The monomer mixture may be added neat or as an emulsion in water. The monomer mixture may be added in one or more additions or continuously, linearly or not, over the reaction period or combinations thereof.
Conventional free radical initiators (oxidants) which may be used in addition to 0.01-1.0%, by weight based on the total weight of the polymer, t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms, preferably 0.01-1.0%, by weight based on the total weight of the polymer, of t-alkyl hydroperoxide wherein the t-alkyl group includes at least 5 Carbon atoms; and more preferably 0.01-1.0%, by weight based on the total weight of the polymer, of t-amyl hydroperoxide include, for example, hydrogen peroxide, sodium peroxide, potassium peroxide, t-butyl hydroperoxide, cumene hydroperoxide, ammonium and/or alkali metal persulfates, sodium perborate, perphosphoric acid and salts thereof, potassium permanganate, and ammonium or alkali metal salts of peroxydisulfuric acid, typically at a level of 0.01% to 3.0% by weight, based on the weight of total monomer. Redox systems using the same initiator(s) coupled with a suitable reductant such as, for example, sodium sulfoxylate formaldehyde, ascorbic acid, isoascorbic acid, alkali metal and ammonium salts of sulfur-containing acids, such as sodium sulfite, bisulfite, thiosulfate, hydrosulfite, sulfide, hydrosulfide or dithionite, formadinesulfinic acid, hydroxymethanesulfonic acid, sodium 2-hydroxy-2-sulfinatoacetic acid, acetone bisulfite, amines such as ethanolamine, glycolic acid, glyoxylic acid hydrate, lactic acid, glyceric acid, malic acid, tartaric acid and salts of the preceding acids may be used. Redox reaction catalyzing metal salts of iron, copper, manganese, silver, platinum, vanadium, nickel, chromium, palladium, or cobalt may be used.
By xe2x80x9cin the presence of 0.01-1.0%, by weight based on the total weight of said polymer, t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atomsxe2x80x9d is meant that the cumulative amount of t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms which has been added to the reaction zone wherein at least some of the monomers are being converted to the emulsion polymer is 0.01-1.0%, by weight based on the total weight of the polymer; optionally wherein at least 95%, preferably the last 95%, by weight of the monomers are being converted to the emulsion polymer; optionally wherein at least 75%, preferably the last 75%, by weight of the monomers are being converted to the emulsion polymer; optionally wherein at least the last 50% by weight of the monomers are being converted to the emulsion polymer; and optionally wherein at least the last 20% by weight of the monomers are being converted to the emulsion polymer. The optional additional oxidant includes those listed hereinabove as conventional free radical initiators such as, for example, tert-butylhydroperoxide, hydrogen peroxide, ammonium persulfate, and the like. In certain embodiments of the present invention, it is advantageous to choose a mixture containing one hydrophilic initiator and the relatively hydrophobic t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms in order to increase the overall efficiency of the initiator system with regard to the initiation of the full range of hydrophilic and hydrophobic monomers; preferably the optional additional oxidant(s) are less than 50% by weight of the total amount of initiator/oxidant. In this embodiment the t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms initator(s) and optional at least one other oxidant may be used as such or as the oxidant component(s) of a redox system using the same initiator(s) coupled with at least one suitable reductant such as those listed hereinabove.
In one embodiment, after 90-99.7%, preferably 95-99.7%, of the monomers by weight, based on the total weight of the polymer, have been converted to polymer, at least half of the remaining monomer is converted to polymer in the resence of 0.01-1.0%, by weight based on the total weight of the polymer, of t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms; preferably in the presence of 0.01-1.0%, by weight based on the total weight of the polymer, of t-alkyl hydroperoxide wherein the t-alkyl group includes at least 5 Carbon atoms; and more preferably in the presence of 0.01-1.0%, by weight based on the total weight of the polymer, of t-amyl hydroperoxide. This part of the reaction may be effected as soon as 90-99.7%, preferably 95-99.7%, conversion of the monomers to polymer is completed in the same reaction vessel or kettle. It may be effected after a period of time, in a different reaction vessel or kettle, or at a different temperature than the preceding part of the polymerization. Preferred is the presence of t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms only after 90%, more preferably only after 95%, conversion of the monomers to polymer is completed.
The t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms, optional additional oxidant(s), and optional reductant(s) may be added, for example, together or separately, in one or more shots or gradually, whether uniformly or not, or in combinations thereof or variations thereon as is desired; they may be added neat, in solution, or emulsified in an appropriate medium.
Chain transfer agents such as, for example, halogen compounds such as tetrabromomethane; allyl compounds; or mercaptans such as alkyl thioglycolates, alkyl mercaptoalkanoates, and C4-C22 linear or branched alkyl mercaptans may be used to lower the molecular weight of the formed polymer and/or to provide a different molecular weight distribution than would otherwise have been obtained with any free-radical-generating initiator(s). Linear or branched C4-C22 alkyl mercaptans such as n-dodecyl mercaptan and t-dodecyl mercaptan are preferred. Chain transfer agent(s) may be added in one or more additions or continuously, linearly or not, over most or all of the entire reaction period or during limited portion(s) of the reaction period such as, for example, in the kettle charge and in the reduction of residual monomer stage.
In another aspect of the present invention the emulsion polymer may be prepared by a multistage emulsion polymerization process, in which at least two stages differing in composition are polymerized in sequential fashion. Such a process usually results in the formation of at least two mutually incompatible polymer compositions, thereby resulting in the formation of at least two phases within the polymer particles. Such particles are composed of two or more phases of various geometries such as, for example, core/shell or core/sheath particles, core/shell particles with shell phases incompletely encapsulating the core, core/shell particles with a multiplicity of cores, and interpenetrating network particles. In all of these cases the majority of the surface area of the particle will be occupied by at least one outer phase and the interior of the particle will be occupied by at least one inner phase. Each of the stages of the multi-staged emulsion polymer may contain the same monomers, surfactants, initiation system, chain transfer agents, etc. as disclosed herein-above for the emulsion polymer. In the case of a multi-staged polymer particle the Tg for the purpose of this invention is to be calculated by the Fox equation as detailed herein using the overall composition of the emulsion polymer without regard for the number of stages or phases therein. Similarly, the amount of acid monomer shall be determined from the overall composition of the emulsion polymer without regard for the number of stages or phases therein. The polymerization techniques used to prepare such multistage emulsion polymers are well known in the art such as, for example, U.S. Pat. Nos. 4,325,856; 4,654,397; and 4,814,373. The emulsion polymer is also contemplated to be formed in two or more stages, the stages differing in molecular weight. Blending two different emulsion polymers is also contemplated.
The emulsion polymer has an average particle diameter from 20 to 1000 nanometers, preferably from 70 to 300 nanometers. Particle sizes herein are those determined using a Brookhaven Model BI-90 particle sizer manufactured by Brookhaven Instruments Corporation, Holtsville N.Y., reported as xe2x80x9ceffective diameterxe2x80x9d. Also contemplated are multimodal particle size emulsion polymers wherein two or more distinct particle sizes or very broad distributions are provided as is taught in U.S. Pat. Nos. 5,340,858; 5,350,787; 5,352,720; 4,539,361; and 4,456,726.
The aqueous composition is prepared by techniques which are well known in the coatings art. First, if the coating composition is to be pigmented, at least one pigment is well dispersed in an aqueous medium under high shear such as is afforded by a COWLES(copyright) mixer. Then the aqueous emulsion polymer is added under lower shear stirring along with other coating adjuvants as desired. Alternatively, the aqueous emulsion polymer may be included in the pigment dispersion step. The aqueous composition may contain conventional coating adjuvants such as, for example, tackifiers, pigments, emulsifiers, crosslinkers, coalescing agents, buffers, neutralizers, thickeners or rheology modifiers, humectants, wetting agents, biocides, plasticizers, antifoaming agents, colorants, waxes, and anti-oxidants.
The solids content of the aqueous coating composition may be from about 10% to about 85% by volume. The viscosity of the aqueous composition may be from 0.05 to 2000 Pa.s (50 cps to 2,000,000 cps), as measured using a Brookfield viscometer; the viscosities appropriate for different end uses and application methods vary considerably.
The aqueous composition may applied by conventional application methods such as, for example, brush or paint roller, air-atomized spray, air-assisted spray, airless spray, high volume low pressure spray, air-assisted airless spray, and electrostatic spray.
The aqueous composition may be applied to a substrate such as, for example, plastic including sheets and films, glass, wood, metal such as aluminum, steel, and phosphate or chromate-treated steel, previously painted surfaces, weathered surfaces, cementitious substrates, and asphaltic substrates, with or without a prior substrate treatment such as a primer.
The aqueous composition coated on the substrate is typically dried, or allowed to dry, at a temperature from 20xc2x0 C. to 95xc2x0 C.